I. Field of the Invention
This invention pertains to a pressurized air system for vehicles and, more particularly, to a control system for regulating air pressure in vehicle air reservoirs.
II. Description of the Prior Art
In the prior art, a particular embodiment of a pressurized air system for vehicles which controls the air pressure in air reservoirs between a predetermined maximum air pressure and a predetermined minimum air pressure, by use of an air governor, is well known. Air governors are commonly used in trucks to control air pressures in air reservoirs which deliver air to service brakes and other air-actuated components.
In a typical embodiment of the prior art as implemented in a truck having a tandem and possibly a trailer, the air governor is connected to a supply reservoir (also known as a "wet tank"). The supply reservoir is an air reservoir connected in parallel with a primary air reservoir and a secondary air reservoir. Generally, one air reservoir is connected only to service brakes on the tandem and the trailer. The other air reservoir may be connected to front brakes, parking brakes, windshield wipers and other air-actuated components.
The air pressure from the supply reservoir enters the air governor and acts on a piston and an inlet exhaust valve. As air pressure in the supply reservoir increases to the predetermined maximum air pressure, an inlet passage of the air governor opens, allowing air pressure to flow through the inlet passage and to an air compressor unloading mechanism. The flow of air to the air compressor unloading mechanism causes the air compressor to stop compressing air. As air pressure in the supply reservoir decreases to the predetermined minimum air pressure, the inlet passage closes and the exhaust opens, allowing air in the unloading mechanism to escape back through the air governor and out of an exhaust port. The release of air from the unloading mechanism actuates the air compressor to compress air and supply compressed air to the supply reservoir and the other air reservoirs.
The air governor is generally mounted to the air compressor, although it can be remotely mounted. It is generally not mounted in the cab due to its noise and size. An example of such an air governor is the Bendix D-2 Governor, No. SD-01-16, Service Date 7/84.
While the air governor such as described has proven to be useful in the industry, it has several disadvantages. When mounted to the air compressor or elsewhere in the engine compartment, the air governor is exposed to excessive vibration from the engine, dirt, extreme temperature fluctuations, moisture and other problematic external parameters. Because of the air governor's numerous moving parts and continued exposure to external parameters while in operation, the air governor fails frequently and unpredictably. When the air governor fails, service brakes can lock up and other air-actuated components can stop working. In addition, maintenance and trouble-shooting are difficult due to the frequent, unpredictable failure of parts and the large number of small high tolerance moving parts within the air governor. Finally, the air governor measures system air pressure from the supply reservoir which receives air directly from the air compressor, rather than measuring the actual air pressure in the air reservoirs that provide air to the air-actuated components. Air pressure from the supply reservoir is not always a reliable measure of air pressure in the primary and secondary reservoirs due to leaks downstream from the supply reservoir.